This invention relates to the manufacture of polycrystalline cubic boron nitride abrasive compacts.
Boron nitride exists typically in three crystalline forms, namely cubic boron nitride (CBN), hexagonal boron nitride (hBN) and wurtzitic cubic boron nitride (wBN). Cubic boron nitride is a hard zinc blende form of boron nitride that has a similar structure to that of diamond. In the CBN structure, the bonds that form between the atoms are strong, mainly covalent tetrahedral bonds. Methods for preparing CBN are well known in the art. One such method is subjecting hBN to very high pressures and temperatures, in the presence of a specific catalytic additive material, which may include the alkali metals, alkaline earth metals, lead, tin and nitrides of these metals. When the temperature and pressure are decreased, CBN may be recovered.
CBN has wide commercial application in machining tools and the like. It may be used as an abrasive particle in grinding wheels, cutting tools and the like or bonded to a tool body to form a tool insert using conventional electroplating techniques.
CBN may also be used in bonded form as a CBN compact, also known as PCBN. CBN compacts tend to have good abrasive wear, are thermally stable, have a high thermal conductivity, good impact resistance and have a low coefficient of friction when in contact with a workpiece.
Diamond is the only known material that is harder than CBN. However, as diamond tends to react with certain materials such as iron, it cannot be used when working with iron containing metals and therefore use of CBN in these instances is preferable.
CBN compacts comprise sintered polycrystalline masses of CBN particles. When the CBN content exceeds 75 percent by volume of the compact, there is a considerable amount of CBN-to-CBN contact and bonding. When the CBN content is lower, e.g. in the region of 40 to 60 percent by volume of the compact, then the extent of direct CBN-to-CBN contact and bonding is less.
CBN compacts will generally also contain a binder containing one or more of phase(s) containing aluminium, silicon, cobalt, nickel, titanium, chromium, tungsten and iron.
A further secondary hard phase, which may be ceramic in nature, may also be present. Examples of suitable ceramic hard phases are carbides, nitrides, borides and carbonitrides of a Group 4, 5 or 6 transition metal, aluminium oxide, and mixtures thereof.
The matrix is defined to constitute all the ingredients in the composition excluding CBN.
CBN compacts may be bonded directly to a tool body in the formation of a tool insert or tool. However, for many applications it is preferable that the compact is bonded to a substrate/support material, forming a supported compact structure, and then the supported compact structure is bonded to a tool body. The substrate/support material is typically a cemented metal carbide that is bonded together with a binder such as cobalt, nickel, iron or a mixture or alloy thereof. The metal carbide particles may comprise tungsten, titanium or tantalum carbide particles or a mixture thereof.
A known method for manufacturing the polycrystalline CBN compacts and supported compact structures involves subjecting an unsintered mass of CBN particles, to high temperature and high pressure conditions, i.e. conditions at which the CBN is crystallographically stable, for a suitable time period. A binder phase may be used to enhance the bonding of the particles. Typical conditions of high temperature and pressure (HTHP) which are used are temperatures in the region of 1100° C. or higher and pressures of the order of 2 GPa or higher. The time period for maintaining these conditions is typically about 3 to 120 minutes.
The sintered CBN compact, with or without substrate, is often cut into the desired size and/or shape of the particular cutting or drilling tool to be used and then mounted on to a tool body utilising brazing techniques.
High CBN materials (also known as PCBN) are used mainly in machining applications such as grey cast iron, powder metallurgy (PM) steels, high chromium cast irons, white cast irons and high manganese steels. High CBN materials are used normally in roughing and heavy interrupted machining operations. In certain cases they are also used in finish machining, such as finish machining of grey cast iron and powder metallurgy (PM) irons.
Such a wide application area for PCBN places a demand for a material that has a high abrasion resistance, high edge integrity, high strength, high toughness, and high heat resistance. These combinations of properties can only be achieved by a material that has high CBN content, at least 75 volume % and a binding phase that will form a high strength bond with CBN.
Because CBN is the most critical component of the high CBN material which provides hardness, strength, toughness, high thermal conductivity, high abrasion resistance and low friction coefficient in contact with iron bearing materials, the main function of the binder phase is to cement the CBN grains in the structure and complement CBN properties in the composite. Therefore, the weaker link in the high CBN composite design is the binder phase as compared to CBN.
U.S. Pat. No. 6,316,094 and EP 1,043,410 both describe methods of making polycrystalline CBN compacts which contain a low, i.e. less than 70 volume percent, CBN content. These CBN compacts differ materially from compacts of this invention in both overall cBN content and in the function or role of the non-cBN matrix. It is well known in the art that high and low CBN content materials are fundamentally different from one another—evidenced by their use in widely divergent applications.
Low CBN content compact matrix material will include both a secondary hard phase and a binder phase, where the secondary hard phase is the dominant material in the matrix. For these compacts, the matrix phase (particularly the secondary hard phase) plays a significant role in determining, in and of itself, the performance of the compact in application. This matrix phase will be present in sufficient quantity (greater than 30 volume percent) to be continuous in two dimensions. In some examples in the patents cited above, the secondary hard phase, binder phase and CBN are subjected to attrition milling. The purpose of this milling is the reduction in size of the brittle secondary hard phase material and the homogenous dispersion of the binder, secondary hard phase particles and CBN particles.
In high CBN content polycrystalline compacts, the CBN plays the dominant role in determining performance in the application. The role of the matrix is chiefly to facilitate reaction bonding between CBN particles, hence cementing them together. The higher CBN content and required formation of a strong cementing bond necessitates that the matrix mixture in high CBN content compacts contains far higher relative quantities of ductile binder phase material. The compact may still contain some level of secondary hard phase material.